The present invention relates to toners and developers which develop a latent charge image in an electro-photographic method and also for electro-static printing; and an image forming method using the same.
The most common image forming method employing an electro-static charge developing method is composed of a developing step in which an electrostatic latent image is formed on the surface of a photoreceptor and a toner image is formed from the electro-static latent image by means of a dry-type developer composed of fine colored powders, a transfer step in which the above-mentioned toner image is transferred onto a recording sheet, such as paper and then successively, a fixing step in which the toner image is fixed onto the recording sheet due to heating or pressing.
In the developing step, development of the electro-static latent image is conducted for forming the toner image. However, all the toner forming the toner image is not transferred onto the recording paper. Usually, a part of toner remains on the photoreceptor. Heretofore, the remaining toner was collected by means of a cleaning device to be discarded. Recently, however, from the viewpoint of economy and environmental concern, an image forming method employing a so-called toner recycling system in which collected toner is returned to the developing device again by means of a toner conveyance screw and is utilized as toner for developing again has been noticed.
On the other hand, in order to continually form favorable copied images, it is necessary that toner has high fluidity and maintains a stable charge property.
As a technology to improve the fluidity of the toner, it is known to add a fluidity agent such as fine silica particles to the colored particles incorporating at least a coloring agent in the binder resin for mixing.
However, if an image is formed by the above-mentioned image forming method employing a toner recycling system in which toner incorporating a fluidity agent having small particle size, the toner receives excessive physical compression force by means of the toner conveyance screw. As a result, the fluidity agent which should exist on the surface of the toner particles is buried into the toner particles. Therefore, the fluidity of toner is gradually reduced, and together with this, the charge amount of toner is changed so that toner splashing and fogging occur. Further, image density is reduced.
Another factor contributing to the durability of a developer is how to continue to stabilize charge performance of the carrier. Causes of the deterioration of the carrier mainly includes abrasion and peeling of the coated resin and so-called toner spent in which fine toner powders adhere on the surface of carrier for contamination.
Compared with a conventional image forming method, in the case of the image forming method employing the toner recycling system, toner receives stirring stress more frequently. Therefore, it is known that toner fine powder is numerously generated and that carrier deterioration due to the toner spent is further promoted.
In order to solve the above-mentioned problems, technologies to remove toner spent substance by the use of a fluidity agent having large particle size as an abrasion agent are disclosed by, for example, Japanese Patent Publication Open to Public Inspection (hereinafter, referred to as JP Nos. 62-180376 and 1-234859).
As described in aforesaid patent applications, when the particle size of the fluidity agent becomes larger, there is a problem that abrasion scratch due to the fluidity agent occurs on the surface of the photoreceptor, causing stain image due to poor cleaning. Specifically, when an image is formed by means of an image forming method employing an organic photoreceptor (OPC), there is a problem that abrasion scratch easily occurs on the photoreceptor caused by the fluidity agent when the organic photoreceptor is soft. Therefore, there is another technology that by the use of a photoreceptor having a high hardness photosensitive layer such as amorphous silicone, abrasion scratch is reduced. However, there are still unsolved problems in that an amorphous silicone photoreceptor has little charge retention ability and that manufacturing cost of the overall apparatus is increased since the amorphous silicone photoreceptor is expensive.
Due to the above-mentioned problems, the following technologies are disclosed in order to enhance abrasion force by using toner.
(1) A toner by mixing inorganic powder, formed by an burning method, whose BET specific surface according to a nitrogen adsorption method is 0.2-30 m.sup.2 /g with nuclei particles (see JP 60-136752). PA0 (2) A toner prepared by mixing inorganic powder whose BET specific surface according to a nitrogen adsorption method is 40-200 m.sup.2 /g and whose average particle size is 0.2-2 .mu.m with a nuclei particle (see JP 4-44053).
However, according to a toner described in item (1) above, though the average particle size of fine inorganic particles is large, its specific area is small. Therefore, though abrasion effect due to fine inorganic powder is considerable, specific area is so small that abrasion area is accordingly small. Therefore, abrasion force by the fine inorganic powder on the surface of carrier is insufficient.
According to a toner described in item (2), by the use of fine inorganic powders having a large average particle size and a large specific area, improvement in abrasion effect can be attained by utilizing unevenness on the surface of fine inorganic powder. However, since the average particle size of the fine inorganic powders is large, the fine inorganic powder releases from the toner frequently. Therefore, if the above-mentioned fine inorganic powder is used for an image forming method employing an organic photoreceptor specifically, the fine inorganic powder receives excessive pressure at the cleaning section. As a result, released fine inorganic particles damage the photoreceptor.